实例化构造函数

从这里可以看出new Vue实际上是使vue构造函数实例化，然后调用_init方法

_init方法,该方法在 src/core/instance/init.js 中定义

Vue.prototype._init = function (options?: Object) {

    const vm: Component = this

    // a uid

    vm._uid = uid++

    let startTag, endTag

    /* istanbul ignore if */

    if (process.env.NODE_ENV !== 'production' && config.performance && mark) {

      startTag = `vue-perf-start:${vm._uid}`

      endTag = `vue-perf-end:${vm._uid}`

      mark(startTag)

    }

    // a flag to avoid this being observed

    vm._isVue = true

    // merge options

    if (options && options._isComponent) {

      // optimize internal component instantiation

      // since dynamic options merging is pretty slow, and none of the

      // internal component options needs special treatment.

      initInternalComponent(vm, options)

    } else {

      vm.$options = mergeOptions(

        resolveConstructorOptions(vm.constructor),

        options || {},

        vm

      )

    }

    /* istanbul ignore else */

    if (process.env.NODE_ENV !== 'production') {

      initProxy(vm)

    } else {

      vm._renderProxy = vm

    }

    // expose real self

    vm._self = vm

    initLifecycle(vm)

    initEvents(vm)

    initRender(vm)

    callHook(vm, 'beforeCreate')

    initInjections(vm) // resolve injections before data/props

    initState(vm)

    initProvide(vm) // resolve provide after data/props

    callHook(vm, 'created')

    /* istanbul ignore if */

    if (process.env.NODE_ENV !== 'production' && config.performance && mark) {

      vm._name = formatComponentName(vm, false)

      mark(endTag)

      measure(`vue ${vm._name} init`, startTag, endTag)

    }

    if (vm.$options.el) {

      vm.$mount(vm.$options.el)

    }

  }

Vue 初始化主要就干了几件事情，传入option，合并option配置，初始化生命周期，初始化事件中心，初始化渲染，初始化 data、props、computed、watcher 等等